In the war on West Nile Virus, mosquito-eating fish are the latest biological weapon

Marc Mitchell places a mosquito fish into a pool behind a vacant house in South Pasadena where rain water has collected and become filled with mosquito larvae on Aug. 9, 2018. (Photo by John McCoy)

Millie Cavafian lives in South Pasadena next door to a vacant home with an old swimming pool teeming with green algae.

“Five minutes out of the house and we are bitten by mosquitoes. We have kids and they have complained about it,” she told Marc Mitchell, vector control specialist.

Marc Mitchell talks to Emily, 5, and Andrew, 3, Cavafian after he found mosquito larva in a fountain in front of their house in South Pasadena on Aug. 9, 2018. (Photo by John McCoy)

At one end of the kidney-shaped pool, rings popped up in the green muck marking where mosquito larvae were jumping up for air.

Mitchell scooped several mosquito fish from his bucket and let them swim free into the murky pool water

“Thank you for addressing that one over there,” Cavafian said to him.

One look at these skinny topminnows being dropped into pools, fountains and flood control channels and you would never know they have a voracious appetite.

But once in the water, they feed like crazy on mosquito eggs, larvae and pupae, a kind of caviar for these nondescript fish that makes them fat and happy.

More importantly, they become deputized militia in the war on mosquitoes that carry life-threatening diseases such as West Nile Virus.

Biological weapons

For vector control districts of Southern California, Gambusia affinis, or mosquito fish, are the guard dogs against mosquito overpopulation and transmission to humans of West Nile virus, St. Louis encephalitis, Dengue Fever, Zika virus and other serious and potentially fatal diseases.

A mosquito fish, or Gambusia affinis. (Handout photo)

More experts on mosquito eradication are turning to mosquito fish as a safer, more cost-effective weapon than chemical larvicides and other pesticides that lose their potency and could harm other fish or crustaceans in nearby waters.

“Using the fish is a more effective solution than putting in pesticides,” said Levy Sun, spokesman for the San Gabriel Valley Mosquito & Vector Control District, which covers 26 cities and 287 square miles. “Ultimately, we want the longest lasting non-pesticide solution.”

Mosquito fish as a biological control mechanism are slowly replacing pesticides such as methoprene (sold as Altosid), which stops the mosquito larvae from maturing into adults, Levy said.

Although studies show methoprene is safe for humans, there’s controversy over evidence from U.S. Environmental Protection Agency studies done in 1996 that says it affects other invertebrates and has been known to kill off frogs, lobsters and certain fish.

Sign up for The Localist, our daily email newsletter with handpicked stories relevant to where you live. Subscribe here.

A safer pesticide is something called BTI, short for Bacillus thuringiensis israelensis, a group of bacteria that release crystals that are ingested by the larvae and rupture their gut, killing them before they reach adulthood in less than 24 hours. BTI does not harm other living creatures in your koi pond, or in creeks, streambeds, marshland or flood channels.

Both larvicides are safe and are “not a direct threat to human health,” Sun wrote in an email. “Our agency rarely uses adult mosquito treatments unless absolutely necessary.”

“You can eat this stuff. It (BTI) is very safe to use ” said Joseph Conlon, a spokesman for the American Mosquito Control Association. “It is is the most largely used larvicide in the world.”

Still, local agencies see Gambusia affinis as a more potent tool that can stretch their budgets as they become inundated with an increasing number of calls about mosquitoes that are spreading rapidly, despite their control efforts.

BTI doesn’t reach all the larvae when in a pool or algae-filled pond with other plant and animal life besides mosquitoes, Conlon said.

Plus, it may only last a week and can be diluted by rainfall or a sprinkler spray, Sun said.

Greater Los Angeles County Vector Control District Treatment spray anti-mosquito larvicide in a 90-acre area south of Garvey Avenue between Santa Anita and Central avenues in South El Monte on early Thursday morning, June 27, 2013. The larvicide is aimed at eradicating the disease-carrying Asian tiger mosquito.(SGVN/Staff photo by Watchara Phomicinda)

Asian tiger mosquito

The Asian tiger mosquito, Aedes albopictus, is a new kind of threat.

It feeds on humans day or night and will bite several times, not getting a full blood meal immediately, Sun said.

The one-quarter-inch long mosquito with black-and-white stripes can transmit chikungunya, a disease that can cause high fevers and extreme muscle and bone aches.

In the African language of Makonde, the name chikungunya means “one who bends up” or curls into a fetal position due to the unbearable pain.

The mosquitos also transmit Zika and Dengue Fever, Conlon said. So far, the only cases of these diseases in Los Angeles County have been imported; no local mosquitoes have carried the virus.

There have been 19 human cases in California of West Nile Virus as of Aug. 10, with four in the county of Los Angeles, one each in Ventura, San Bernardino and Orange counties and two in Riverside County, according to the state Department of Health and other health agencies.

Los Angeles County has the most human cases of any county in California, according to the state.

“The population exploded in the San Gabriel Valley,” Sun said, adding that eradicating Aedes is a much more labor intensive task.

Mosquito fish are a long-term approach to reducing the mosquito population. They can stay in an old pool or pond for up to a month eating larvae, he said.

BTI or methoprene used to fight Aedes require repeated applications as effectiveness gets diluted by rain or other water sources.

“So that is why we are ramping up our mosquito fish program,” he said. “By having more mosquito fish, it will reduce the amount of visits (by technicians) to the places we put the fish and that gives us the opportunity to look at other places, such as flood control channels.”

From pool to fountains

On the way back to this truck, something caught Mitchell’s eye: a half-filled ornamental fountain sitting under the eaves of Cavafian’s home, collecting water from the lawn sprinklers.

Mitchell scooped out the water and any larvae. He suggested Cavafian make a plant holder out of the old fountain by drilling holes in it and filling it with potting soil.

A chagrined Cavafian said she told her husband about the old fountain for several weeks.